Many eukaryotic cells are enveloped by an extracellular matrix of proteins that provide structural support, cell and tissue identity, and autocrine, paracrine and juxtacrine properties for the cell within its environment. (McGowan, S. E. (1992) FASEB J. 6:2895-2904.) The diverse biochemistry of extracellular matrix (ECM) proteins is indicative of the many, often overlapping, roles that are attributed to each distinct molecule. (Grant. D. S. and Kleinman, H. K. (1997) EXS 79:317-333.) Whilst a great number of ECM proteins have been isolated, it remains unclear how the majority of ECM proteins interact with one another or with other molecules residing within the cell membrane.
Many ECM proteins have been associated with tissue growth and cell proliferation, others with tissue or cell differentiation, and yet others with cell death. (Taipale, J. and KeskiOja, J. (1997) FASEB J. 11:51-59; and Eleftheriou, C. S. et al. (1991) Mutat. Res. 256:127-138.) For example, the process of embryonic bone formation involves the creation of an extracellular matrix that mineralizes during the course of tissue maturation. During the life of an individual, this matrix is subject to constant remodeling through the combined actions of osteoblasts, which form mineralized bone, and osteoclasts, which resorb bone. The balance of ECM composition, and the resulting bone structure, may be perturbed by biochemical changes that result from congenital, epigenetic, or infectious diseases. (Francomano, C. A. et al. (1996) Curr. Opin. Genet. Dev. 6:301-308.)
ECM proteins also act as important mediators and regulators during the inflammatory response. Leukocytes are primed for inflammatory mediator and cytokine production by binding to ECM proteins during extravasation. (Pakianathan. D. R. (1995) J. Leukoc. Biol. 57:699-702.) Deposition of ECM proteins is also triggered by inflammation in response to lung injury. Although the function of newly deposited matrices in injured lungs is unknown. their ability to affect the migration, proliferation, differentiation, and activation state of cells in vitro suggests an important role in the initiation and maintenance of the inflammatory response in vivo. (Roman, J. (1996) Immunol. Res. 15:163-178.)
Multidomain or mosaic proteins play an important role in the diverse functions of the ECM. ECM proteins are frequently characterized by the presence of one or more domains which may include collagen-like domains, EGF-like domains, immunoglobulin-like domains. fibronectin-like domains, and von Willebrand Factor A-like modules. (Ayad, S. et al. (1994) The Extracellular Matrix FactsBook, Academic Press, San Diego, Calif. pp 4-7.)
Cell adhesion molecules are located in the plasma membrane and associate with the ECM. They have been shown to stimulate axonal growth through homophilic an/or heterophilic interactions with other molecules. For example, proteins that contain the Arg-Gly-Asp (RGD) attachment site, and the integrins that serve as their receptors, constitute a major recognition system for cell adhesion. In addition, interactions between adhesion molecules and their receptors can potentiate the effects of growth factors upon cell biochemistry via shared signaling pathways. (Ruoslahti, E. (1997) Kidney Int. 51:1413-1417.)
Integrins are ubiquitous transmembrane adhesion molecules that link cells to the ECM by interacting with the cytoskeleton. Integrins also function as signal transduction receptors and stimulate changes in intracellular calcium levels and protein kinase activity. (Sjaastad, M. D. and Nelson W. J. (1997) BioEssays 19:47-55.) For example, fibronectin is recognized by at least ten cell surface receptors of the integrin family which mediate the involvement of fibronectin in many different biological processes.
The composition of the ECM is also regulated by differential proteolytic activity. Cysteine proteases (e.g., cathepsin) are produced by monocytes, macrophages and other immune cells and are involved in diverse cellular processes ranging from the processing of precursor proteins to intracellular degradation. Overproduction of these enzymes can cause the tissue destruction associated with rheumatoid arthritis and asthma.
Smith-Magenis syndrome (SMS) is a multiple congenital anomaly and mental retardation syndrome associated with the deletion of human chromosome 17p 11.1. In normal tissue, chromosome 17p11.1 contains a gene encoding microfibril-associated glycoprotein 4 (MFAP4) in addition to several other genes which are homologous to known eukaryotic proteins. (Chen, K. S. et al. (1995) Am. J. Hum. Genet. 56:175-182; and Koyama, K. et al. (1996) Cytogenet. Cell. Genet. 72:78-82.) The MFAP4 gene is deleted in almost all SMS patients studied. MFAP4 has a fibrinogen-like domain and the N-terminus has an RGD sequence which suggests that the gene encodes an ECM protein involved in cell adhesion or intracellular interactions. (Zhao, Z. et al. (1995) Hum. Mol. Genet. 4:589-597.) Furthermore. the gene encoding MFAP2, a candidate gene for involvement in the etiology of inherited connective tissue diseases, contains two alternatively used 5' untranslated exons. (Faraco, J. et al. (1995) Genomics 25:630-637.)
The discovery of a new human microfibril-associated glycoprotein 4 splice variant and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of developmental, reproductive. muscle, immunological, and neoplastic disorders.